Method of making an ion responsive electrode

ABSTRACT

A solid state pH measuring electrode having the pH measuring electrode structure formed by successive layers on an insulating substrate with an outer pH sensitive glass layer being deposited on a supporting solid electrolyte layer by RF sputtering. The reference electrode is similarly formed by depositing an outer layer of glass onto a supporting solid electrolyte layer by RF sputtering with the temperature expansion of the glass and supporting solid electrolyte structure being selected to produce a differential expansion causing random cracking of the glass layer during temperature cycling of the reference electrode. A combination structure is provided wherein the pH measuring electrode and the reference electrode are formed on opposite sides of the same electrically insulating substrate with a thermal compensating element being included in the integrated package.

CROSS REFERENCE TO COPENDING APPLICATION

The present application is a division of application Ser. No. 661,958,filed on Feb. 27, 1976, now U.S. Pat. No. 4,031,606, which is a divisionof application Ser. No. 552,284 filed on Feb. 24, 1975, now abandonedupon the filing of continuation application Ser. No. 666,166 on Mar. 11,1976 and assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ion concentration measuring apparatus.More specifically, the present invention is directed to a solid stateion responsive electrode and reference electrode.

2. Description of the Prior Art

Conventional ion concentration measuring electrode structures haveusually used a glass measuring electrode, a reference electrode and athermal compensator. For example, various types of special glasses havebeen used to measure the pH of aqueous solutions. In making these glasselectrodes the pH sensitive glass is usually fused to the end of a lessexpensive glass tube and is subsequently blown into a small bulb ofabout 2 to 4 mils thick. These "hand-blown" pH glass electrodes arefragile, have very high electrical impedance due to the thickness of theglass and are used for limited temperature ranges mainly because of theinternal pressure developed by a liquid electrolyte fill which issubsequently introduced into the interior of the pH measuring electrodeto provide an electrically conductive ion source. An example of atypical prior art pH electrode apparatus is shown in U.S. Pat. No.3,405,048 of D. J. Soltz. These prior art glass electrodes are expensivemainly because of the extensive use of highly skilled manual labor inthe construction of the glass envelope and the subsequent fillingthereof. A somewhat similar construction is used in the construction ofthe prior art reference cell which additionally increases the cost ofthe overall conventional pH measuring system. Despite its disadvantages,the glass electrode has retained its popularity in the field of ionconcentration measurement even after attempts to develop a solid stateelectrode such as that shown in U.S. Pat. No. 3,498,901 of L. T. Metz etal since the response of the glass electrode is faster than other priorart devices with the glass electrode also having the broadest useful pHrange. However, in order to provide a low cost and even more useful ionconcentration measuring system it is desirable to produce a lowimpedance, high reliability and relatively unbreakable ion concentrationmeasuring electrode and reference electrode.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved solid stateion responsive and reference electrode structure. Another object of thepresent invention is to provide a method for making an ion responsiveelectrode structure.

In accomplishing these and other objects, there has been provided, inaccordance with the present invention, a method for making a combinationion responsive and reference electrode structure having an insulatingsubstrate supporting an electrically conductive structure overlaid witha solid electrolyte layer having a final thin layer of ion responsiveglass being attached to the solid electrolyte layer by RF sputtering. Inthe reference electrode, the outer glass layer has a coefficient ofthermal expansion different from the solid electrolyte layer while inthe ion responsive pH electrode, the outer ion responsive glass layer issubstantially thermally matched to the supporting electrolyte layer.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention may be had when thefollowing detailed description is read in connection with theaccompanying drawings, in which:

FIG. 1 is a pictorial illustration of a cross-section of a referenceelectrode embodying the present invention,

FIG. 2 is a pictorial illustration of a cross-section of an ionconcentration measuring electrode embodying the present invention, and

FIG. 3 is a pictorial illustration of a cross-section of a combinationion responsive, reference electrode and thermal compensator embodyingthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 in more detail, there is shown a pictorialillustration of a cross-section of a reference electrode embodying thepresent invention. An electrically insulating substrate 2 of anelectrically insulating material, e.g., ceramic or glass, has a firstlayer 4 of chromium (Cr) deposited on one side thereof. A second layer 6of silver (Ag) is subsequently deposited on the chromium and a thirdlayer 8 of silver chloride (AgCl) is deposited on the silver. An outerlayer 10 of a suitable glass is RF sputtered on the exposed surface ofthe silver chloride layer 8. The glass material for the outer layer 10is selected to have a coefficient of thermal expansion different fromthe supporting silver chloride structure 8 whereby a subsequenttemperature cycling of the multilayer structure is effective to producemicroscopic cracks in the outer glass layer. For example, borosilicateglass has a coefficient approximately one tenth that of silver chloride.These cracks provide ion conduction paths to the silver chloride layerfrom an aqueous solution in which the reference electrode is immersedduring pH measurements. A socket shell 12 is arranged to enclose asingle contact pin 14. The contact pin is electrically connected by anelectrically conducting wire 16 to the silver layer 6 of the multilayerstructure. An encapsulating, or potting, compound 18 is subsequentlyapplied to the multi-layer structure, the socket shell 12, the contactpin 14 and the connecting wire 16 to form a moisture-proof barrier andto unite the elements into a rigid package. An open window, or hole, 19is formed through the potting compound 18 to expose the outer glasslayer 10.

In FIG. 2, there is shown a pictorial illustration of a cross-section ofan exemplary pH measuring electrode embodying the present invention.Similar reference numbers have been used in FIGS. 1 and 2 to indicatesimilar structural elements although the combination of FIG. 2 isdirected toward a different device from that shown in FIG. 1. Anelectrically insulating substrate 2, e.g., glass or ceramic, is used asa support member for a multi-layer structure similar to that used in thereference electrode. Specifically, the glass substrate 2 is first platedwith a first layer 4 of chromium which is followed by a second layer 6of silver and a subsequent third layer 8 of silver chloride. An outerlayer 20 of pH sensitive glass is then RF sputtered on the silverchloride layer. The temperature coefficient of the silver chloride andpH glass layer are matched whereby the pH glass will not producemicroscopic cracks as during normal temperature cycling, e.g., 0° to100° C, as in the case of outer glass layer used in the referenceelectrode previously described. For example, Corning 1990 glass has acoefficient of thermal expansion approximately one-half that of silverchloride. Other pH sensitive glasses can be produced to even moreclosely match the coefficient of thermal expansion of the silverchloride layer by using glass formulas with the followingcharacteristics: a high coefficient of expansion can be achieved byusing oxides such Li₂ O, Na₂ O, K₂ O, Rb₂ O, Cs₂ O, BaO and SrO while alow coefficient of expansion can be achieved by using SiO₂, B₂ O₃, Al₂O₃, BeO and TiO₂. Thus, the coefficient of thermal expansion of thesilver chloride layer or other solid state electrolyte materials such asCuO, AgI, AbAg₄ I₅, etc. can be matched to an even closer approximationif either the temperature cycling during the measurement operation orthe electrolyte material layer imposes a need for such a match. Thethickness of the pH glass is approximately 10 to 10,000 A. A thermalcompensator structure may be produced using an insulating substrate witha thermal sensitive element mounted therein and having the same overallconfiguration as that used for the aforesaid reference and pH electrodeswhereby the three elements would be used concurrently as shown in theaforesaid Soltz U.S. Pat. No. 3,405,048 patent.

In order to further utilize the solid state nature of the electrodes ofthe present invention, a combination structure having the referenceelectrode, the ion concentration measuring electrode and the thermalcompensator integrated therein is shown in FIG. 3. As in the case ofFIGS. 1 and 2, similar reference numbers have been reused in FIG. 3 forcommon, or similar, elements of the structure but a capital "A"reference letter has been added to some repeated reference numbers toindicate similar elements in adjacent sections of the integrated cellstructure shown in FIG. 3. Thus, in the case of a pH electrode, a firstsubstrate 2 of an electrically insulating material has the firstchromium layer 4 followed by the silver layer 6 and the silver chloridelayer 8 with an outer layer of the selected mismatched temperaturecoefficient glass 10 to form the reference electrode portion of theintegrated cell. A second electrically insulating substrate 2A has achromium layer 4A followed by a silver layer 6A and a silver chloridelayer 8A with a pH glass outer layer 20. A socket shell 24 which mayadvantageously be a larger size than the socket shell 12 shown in FIG. 1and 2 to accommodate an additional number of connector pins is providedadjacent to one side of the aforesaid multilayer structure.

A plurality of electrical connector pins 26, 28, 30 and 32 are locatedwithin the connector shell 24. A first one of the pins 26 is connectedto the silver layer 6A in the pH measuring electrode section of theintegrated multilayer structure by wire 16A. Similarly, the fourth pin32 is connected by a wire 16 to the silver layer 6 in the referenceelectrode portion of the integrated multilayer structure. The second andthird pins 28 and 30 are connected to a thermal compensator element 34by separate wires 36 and 38 whereby the thermal compensator element 34is electrically connected across the second and third pins 28 and 30.The thermal compensator element 34 may be formed in a recess of thesecond substrate element 2A by any suitable means which can include thesame RF sputtering technique used to provide the layers of the pH andreference electrodes structures. Finally, an outer shell, or convering,of a potting compound 40 is provided to enclose the multilayer structureand to secure the pins 26 to 32 while engaging the connector shell 24. Afirst hole, or window, 19 is provided in the covering 40 to expose theglass layer 10 of the reference electrode while a second opening 19A isprovided in the covering 40 to expose the pH glass layer 20 of the pHelectrode structure.

MODE OF OPERATION

Since, in the RF sputtering process operation, the operatingtemperatures are below 200° C, the preparation of the ion responsiveelectrode structure including the ion responsive glass layer isperformed over a much smaller temperature range which further preventsthe ion responsive glass from cracking when it is cooled down to roomtemperature even if the ion responsive glass layer and electrolyte layerdo not have an exact temperature coefficient match. Additionally, thethin, i.e., 10,000 A maximum, glass layer will stretch instead ofcracking during temperature cycling to enable the overall multilayerstructure to withstand temperature cycling over a relatively widetemperature range, e.g., -70° C to +200° C. Inherently, the integratedelectrode structure has a low impedance due to the thinness of the outerglass layer. Another feature is an extreme ease of replacement wherebythe pH electrode, the reference electrode and the thermal compensatorcan be replaced as a single inexpensive unit. Further, since thedelicate glass handling operations required for prior art electrodeshave been eliminated, the high manufacturing repeatability of theproduce and the reduction of manufacturing rejects enhances the lowmanufacturing costs of either the separate electrodes shown in FIGS. 1and 2 or the combinational electrode structure shown in FIG. 3. Finally,in addition to savings in the amount of materials used for the thinlayers of the multilayer structure, additional savings will be effectedby the elimination of certain expensive metals which were necessary inprevious glass electrodes because of the required glass-to-metal seals,e.g., platinum or other similar thermal property metals.

Accordingly, it may be seen that there has been provided, in accordancewith the present invention, a solid state ion responsive and referenceelectrode structure and method having application in either a separateor a combination electrode construction.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method for making a pHelectrode comprising the steps of depositing a metallic layer of anelectrically insulating substrate, depositing a solid electrolyte layeron said metallic layer, and depositing a pH glass layer by RF sputteringon said electrolyte layer, said glass layer having a coefficient ofthermal expansion substantially matched with respect to a coefficient ofthermal expansion of said electrolyte layer to maintain the integrity ofsaid glass layer over a predetermined temperature range.
 2. A compoundof making a pH electrode as set forth in claim 1 and including thefurther steps of providing an electrical connection to said metalliclayer and encapsulating said substrate, said metallic layer and aportion of said pH glass layer while exposing said electricalconnection.
 3. A method of making an ion responsive electrode comprisingthe steps of depositing a metallic layer on an electrically insulatingsubstrate, depositing a solid electrolyte layer on said metallic layerand depositing an ion responsive layer by RF sputtering on saidelectrolyte layer, said ion responsive layer having a coefficient ofthermal expansion substantially matched with respect to a coefficient ofthermal expansion of said electrolyte layer to maintain the integrity ofsaid ion responsive layer over a predetermined temperature range.
 4. Amethod of making an ion responsive electrode as set forth in claim 3 andincluding the further steps of providing an electrical connection tosaid metallic layer and encapsulating said substrate, said metalliclayer and a portion of said ion responsive layer while exposing saidelectrical connection.